Is it the possibility that interest rates could rise? The collapse of the home construction business in America? Alan Greenspan's musings on the possibilities of recession? Or if none of these, what?
The answer at this point is that it doesn't really matter. As many investors, and particularly the very big players, have borrowed money to increase their exposure to shares, a crisis like this tends to kick off a spiral of selling. To stem losses, traders rush to sell to cover their borrowings, which then pushes the market down even further. Stocks end up falling because they are falling.
That said, this time round, there are plenty of fundamental reasons for the falls: stocks in many emerging markets -India and China in particular -look expensive.
Even in western markets prices were beginning to look a bit high. Also the risks of a recession in America really can't be written off any more. January's year-on-year GDP rise came in at a mere 2.2%, having averaged well over 3% in 2006.
I have mentioned all this before, and it doesn't change any of my favourite investment themes. I still think there are better ways to invest in Chinese growth than to invest in the Chinese stock market -getting into the Taiwanese market, for example, or buying exchange-traded commodities.
And I still think the US stock market is a bad place to put your money; that the Indian stock market is a fabulous long-term investment; that in the UK you should move out of pricey mid-caps and into either income-producing blue chips or the many still undervalued small firms on AIM; and that in Japan the resurgence of the property market hints at the beginning of a new bull run.
So with that out of the way, I want to talk about something completely different - tractors.
One of the biggest problems faced by the companies in the commodities boom has been getting their hands on equipment. Take the big miners. It's all very well everyone clamouring for their metals, but this doesn't do them much good if they can't get those metals out of the ground.
A few years ago it took about three months to get a large-haul truck delivered to your mine. Today it takes over two years, and the odds are that when it does turn up it will arrive without its large tyres -because there's a massive shortage of those too.
It's the same story with grinding mills, power generators, crushers, and ship loaders.
Then look at the oil producers and explorers. They can't get their hands on new rigs for love nor money. Rental rates are the highest they have ever been, at up to $500,000 (£ 255,000) a day, and the wait for really deep drilling rigs is three years. Five years ago there were 18 new rigs on order. Now there are 115.
The silver lining, of course, is that the suppliers of all this equipment are making a fortune. So, back to tractors.
I have written before that I see the next stage of the commodities bull market as being a massive rise in the price of soft commodities. Prices have risen noticeably in the past year or so, but the real price of most agricultural products is still not far off historical lows.
The soft-commodity story is similar. While prices have been low there has been no reason to invest in production, with the predictable result that global stocks are now far too low. Yet demand is rising fast, partly due to rising food consumption in the developing nations but also thanks to the ethanol boom which is gobbling up ever larger percentages of the world's corn and sugar.
Some analysts see corn prices rising by 300% or 400% in the next five years, which would divert acreage from other crops such as wheat and soybeans and push up their prices too.
Farmers will then rush to push up their yields. They will buy fertiliser and they will finally start to replace their ageing farm equipment. It won't be long before the wait for a new tractor tyre is not 10 minutes but two years.
So just as we should have been buying oil rigs in 2000, now is the time to invest in tractors, ploughs and combine harvesters.
The obvious choice here is US machinery giant John Deere, but Hugh Hendry of Eclectica Asset Management, a hedge fund, suggests a few more daring picks. After a tractor, a sprayer is the most used tool on a farm, said Hendry, something that makes the "world leader in sprayers", Exel Industries, worth a look.
The firm is currently making lowish margins and trading on a price-earnings ratio of 12 times, but with demand rising fast, margins should soon be pushed back up and the p/e will fall accordingly.
Hendry's next pick -and this is one for the brave -is Norway's Kverneland, which has had a horrible decade so far. It is constantly restructuring and its profitability has collapsed. So why buy it? Because its order book is up 47% on this time last year and it is building a new low-cost plant in Russia. Back in the 1990s, said Hendry, the firm made double-digit margins. It should do so again, making now a good time to buy in.
Editor
Moneyweek Magazine
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Merryn Somerset Webb is a former stockbroker and now editor of Money Week. Her views are personal and investors should always seek professional advice.